Compared to other intelligent communication technologies like the smart antenna, cognitive radio (CR) mitigates the interference by sensing the spectrum and using idle channels. In 2004, when Qualcomm analyzed the feasibility of using CR in cellular wireless communications, it was found that in order to accurately avoid interference the cognitive transmitter is required to measure the effect of its transmission on all possible receivers. However, it is not an easy task for the transmitter to sense the environment of the entire set of possible receivers when the receiver distribution is geographically large. Diversely, even if the transmitter is able to do so, it might be difficult to find the optimal solution for both the transmitter and the receiver without any interference to primary users in such a large area and under such complicated conditions. CR is a self observing, self learning and self decision making radio. When it is performing as the sole secondary user, it is efficient and can reach optimal utilization of the resources. However, when multiple secondary users exist, the competition among the secondary users is a waste of resources.
Traditional communication systems like the Global System for Mobile Communications (GSM) and the Code Division Multiple Access (CDMA) function based on pre-defined channel allocation and sets of protocols. Whereas, CR adapts to channel conditions using the process of sensing an existing wireless channel, evolving a radio's operation to accommodate the perceived wireless channel, and evaluating what happens when a change is made.